Many of the new soft looks in ready-to-wear today are created by cutting the fabric on the bias. A bias-cut garment molds to the shape of the body and emphasizes curves.
If you like the drape of bias but would like to conceal figure faults, you might consider lining or underlining a bias-cut garment. An underlining gives body to the fashion fabric without interfering with its draping qualities. A lined garment will also retain its free flow and won't hang unflatteringly close to the body. When lining or underlining a bias garment, it is vital that all pieces are cut on the true bias.
Often a designer will put a bias-cut sleeve in a straight-grain garment. A bias sleeve is considerably more comfortable than a straight grain sleeve since it gives as the arm moves. A bias sleeve also drapes more gracefully, especially important in a very full sleeve.
Some fabrics are simply too firm to be flattering if cut on the straight grain _ for example, a heavy wool tweed. If the firm fabric is cut on the bias it takes on a softer quality and will mold better to the body.
A straight-grain pattern can be converted easily to a bias-grain pattern. Begin by drawing a line perpendicular to the lengthwise grain line indicated on the pattern. This new line indicates the crosswise grain. Now draw a line equidistant from the lengthwise and crosswise grainlines that also runs through the original intersection point. The angle produced is 45 degrees. Draw one more line, this one perpendicular to the last line drawn. These last two lines indicate the bias grains, and you can use either one.
Use a new bias line as your grainline in the layout, placing the pattern so that the new bias line is parallel to the selvage. Matching plaids on the bias results in a chevron, two sets of stripes which meet at identical angles. If you are planning a plaid bias skirt, choose an even plaid without a pronounced diagonal weave.
Chronicle Features, 1994